This application claims priority of French Patent Application No. FR 00 16142 filed Dec. 12, 2000.
1. Field of the Invention
The present invention relates to a bearing assembly intended to guide the rotation of a helicopter rear transmission shaft. In the production of such bearing assemblies it is necessary to take account of their number, their radial rigidity, in order to afford transmission which is subcritical or supercritical in terms of bending, and also to take into account their axial rigidity which governs the dynamic adaptation in this direction, once again in subcritical or supercritical mode. The bearing assembly has also to be suited to the type, relubricatable or otherwise, of rolling bearing used, to its method of alignment and also to the method of balancing the shaft carried by this bearing assembly.
2. Description of the Related Art
Bearing assemblies consisting of rolling bearings fixed, on the one hand, to a shaft by means of a rubber sleeve and, on the other hand, to a bearing assembly support by means of a rubber ring held on the structure of the craft by means of a strap or of a shaped metal sheet are already known. In other bearing assemblies, the rolling bearing is fixed by its inner race to a flange belonging to the shaft and by its outer race to a ball swivel immobilized using PTFE rings and making it possible to compensate, upon assembly, for any alignment defect the bearing assembly might exhibit. This outer race may also be fixed to a bearing assembly connected to a bearing assembly support by damping sleeves.
The disadvantages with bearing assemblies that have rubber rings lie in the introduction of an additional flexibility, the bearing assemblies thus becoming relatively stiff in the radial direction but more flexible in the axial direction, which means there is nothing actually opposing slippage in this direction or, subsequently, guaranteeing a precise reference along the axis of the transmission. This represents a definite drawback on a helicopter with a rear fenestron. The bearing assembly of the invention must instead provide the desired rigidities, both in the radial and axial directions, and to do so with satisfactory control over the clearances.
It is an object of the present invention therefore to obtain a true axial reference and good dynamic adaptability of the bearing in the axial and radial directions, with high stiffnesses so as to obtain subcritical transmission, and to do so with the bearing assembly being to a certain extent tolerant of defects in the orientation of the bearing supports. It is also desirable to achieve ease of maintenance with the possibility of greasing the rolling bearing and checking its tough spots.
To this end, a bearing assembly for a helicopter rear transmission shaft comprising a rolling bearing fixed by its inner race to the said transmission shaft and means for connecting its outer race to the structure of the craft, is, according to the present invention, characterized in that the said rolling bearing is of the relubricatable type and is mounted tightly on the said shaft by its inner race in such a way as to be prevented from rotating and translating on the shaft, the latter for this purpose comprising a thrust shoulder, in that the outer race of the rolling bearing is shaped as a ball swivel or housed in a ball swivel with a spherical outer peripheral surface consisting of two half ball swivels back to back, and in that the said ball swivel is itself mounted in the spherical housing of a support consisting of two flanges arranged facing one another and slightly separated from one another in the axial direction prior to clamping, namely a support flange which can be fixed by lugs to the said structure of the craft and an adjusting flange that can be fixed to the previous one using screws.
It is also possible to provide elastic O-rings for centering the two half ball swivels on the outer race of the rolling bearing, these rings also holding the half ball swivels during an operation of setting up the rolling bearing.
Thus, the two half ball swivels make it possible, during assembly, for the operation of setting up the rolling bearing to compensate for the defect of orientation of the support and also to avoid excessive loading of the rolling bearing, and to do this without the need to use shims or to carry out an adjustment of the alignment. Furthermore, the elastic rings make it possible to avoid the disadvantage of the outer race of the rolling bearing being mounted tightly in its ball swivel, as this would be incompatible with the small internal clearance of the rolling bearing. As far as the adjusting flange is concerned, it will make it possible to immobilize the outer race of the rolling bearing once the orientation of the rolling bearing has been adjusted and the aforementioned screws on the support flange which provides the connection with the structure have been tightened. Prior to tightening, the axial spacing between the two flanges is between defined lower and upper limits; the tightening of the screws which fix the said adjusting flange to the support flange provides control over the rotational immobilization of the rolling bearing and control over the stresses acting on the flange. This tightening is performed until such point as the two flanges make contact with one another, or without them making contact, depending on whether the adjusting flange is a split flange or a non-split flange, as will be seen more clearly later on.
In principle, the use of the half ball swivels makes it possible to avoid compulsory use of a rolling bearing with a spherical outer race, although this is still possible, as was envisaged above, in which case the bearing assembly would amount simply to such a rolling bearing with a spherical outer race mounted in the spherical housing of the two-flange support.
According to another provision of the present invention, it is also possible to envisage the bearing assembly for a transmission shaft comprising on the one hand, an intermediate ring which can seize under the effect of heat and is mounted between the ball swivel and the outer race of the bearing, the outside and inside diameters of this intermediate ring having a calibrated eccentricity defect and, on the other hand, an accelerometer connected to a system for capturing the imbalance and misalignment of the shaft.
Thus, in the case of degradation to the rolling bearing, the increase in the temperature leads to seizure of the ring and thereafter to expansion of the assembled parts. As these expansions are differential because of the different nature of the materials of which these parts are made (dural in the case of the ball swivel and the flanges, steel in the case of the rolling bearing and, for example, titanium in the case of the intermediate ring), they will cause the rolling bearing to become immobilized in this ring and release it in its housing. The shaft will then rotate about the outside diameter of the intermediate ring, with an eccentricity defect that gives rise to a calibrated imbalance detected by the capture system, which provides warning that the rolling bearing has become degraded.
In an alternative form, the bearing assembly for a transmission shaft comprises, on the one hand, an intermediate ring which can seize under the effect of heat and is mounted between the ball swivel and the outer race of the bearing, the outside and inside diameters of this intermediate ring having a calibrated misalignment and, on the other hand, an accelerometer connected to a system for capturing the imbalance and misalignment of the shaft.
In this case, rotation after the bearing has become immobilized in the intermediate ring will be with a misalignment which is also detected by the capture system.
Advantageously, the said intermediate ring is made of titanium and is coated at its periphery with a fabric based on a material with a low coefficient of friction, for example PTFE.